1. Field of the Invention
The present invention relates to a gain equalizer for equalizing the gain of an optical amplifier. More particularly, the present invention relates to a gain equalizer which includes a plurality of optical filters with transparency characteristics represented by periodic waveforms having different periods.
2. Description of the Related Art
FIG. 1 is a diagram illustrating a conventional optical communication system which uses wavelength division multiplexing (WDM) to increase the transmission capacity of the system. Referring now to FIG. 1, a plurality of optical sending stations (OS) 1000#1 to 1000 #n produce individual signals having different wavelengths of xcex1 to xcexn, respectively, where n is an integer. The individual signals are provided to a multiplexer (MUX) 1002 which multiplexes the individual signals together into a single WDM signal provided to a transmission line 1004. Transmission line 1004 is often a single optical fiber.
The WDM signal propagates through transmission line 1004 and is received by a demultiplexer (DEMUX) 1006. Demultiplexer 1006 demultiplexes the WDM signal light back into individual signals having different wavelengths of xcex1 to xcexn, respectively. Each individual signal is then provided to a corresponding optical receiving station (OR) 1008#1 to 1008#n.
When transmission line 1004 is relatively long (that is, the WDM signal is to be transmitted over a long distance), a plurality of repeaters 1012 must be inserted into transmission line 1004 to amplify the WDM signal as the WDM signal travels through transmission line 1004. A repeater is typically referred to as a xe2x80x9csubmarinexe2x80x9d repeater when it is for use underwater in a transmission line extending, for example, between continents.
Each repeater 1012 typically includes an optical amplifier 1014, which is often an erbium-doped fiber amplifier (EDFA), to amplify the WDM signal. Generally, an EDFA uses an erbium-doped fiber (EDF) as an amplifying medium. As the WDM signal travels through the EDF, pump light is provided to the EDF from a pump light source (not illustrated) so that the pump light interacts with, and thereby amplifies, the WDM signal.
An EDFA has gain versus wavelength characteristics based on the composition of an optical fiber base material used to make the EDF. These gain versus wavelength characteristics are not perfectly flat in a wavelength band of 1.5 to 1.6 xcexcm, the band generally used for long distance optical transmission. Therefore, an EDFA typically experiences an undesirable xe2x80x9cgain tiltxe2x80x9d, where the individual signals in the WDM signal are amplified with different gains, depending on the power of the pump light. For example, when the power of the pump light is relatively high, the EDFA may produce a negative gain tilt, where higher wavelength components in the WDM signal are amplified less than lower wavelength components in the WDM signal. Similarly, when the power of the pump light is relatively low, the EDFA may produce a positive gain tilt, where higher wavelength components in the WDM signal are amplified more than lower wavelength components in the WDM signal. Thus, the gain tilt of an EDFA may not be flat.
When a transmission line extends for a relatively long distance (such as, for example, between continents), it is usually necessary to insert tens of stages of submarine repeaters in series into the transmission line. Therefore, the WDM signal will be amplified by tens of stages of optical amplifiers in series. Unfortunately, when a WDM signal is transmitted through tens of stages of optical amplifiers, the cumulative effect of gain tilt in the optical amplifiers will cause dispersion of optical signal-to-noise ratios (SNRs) of the individual signals in the WDM signal. Such dispersion will result in a low optical SNR which will be further degraded in each subsequent repeater.
For example, FIG. 2(A) is a graph illustrating an optical spectrum waveform where a WDM signal is conventionally transmitted through ten (10) repeaters in series, and FIG. 2(B) is a graph illustrating an optical spectrum waveform where a WDM signal is conventionally transmitted through sixty (60) repeaters in series. In both cases, an Al-low-density (less than 1 Wt %) EDF is used, and the WDM signal includes four individual signals at different wavelengths multiplexed together.
As shown in FIG. 2(A), in the case of ten (10) repeaters in series, the dispersion of optical SNR is relatively small. However, as shown in FIG. 2(B), in the case of sixty (60) repeaters in series, the dispersion of optical SNR is increased and thereby results in individual signals having an insufficient optical SNR.
Various methods have been proposed to compensate for the dispersion of optical SNR when a large number of repeaters are inserted into a transmission line. For example, one such proposed method is to use an Al-low-density (less than 1 Wt %) EDF and a Fabry-Perot etalon optical filter as a gain equalizer. See Takeda, et al., xe2x80x9cGain equalization of Er-doped fiber amplifier using etalon filterxe2x80x9d, a publication of autumn communication society by the Institute of Electronics, Information and Communication Engineers, 1995, B-759.
A second proposed method allows the transmission of twenty (20) waves on a 6300-km path using an Al-low-density EDF and a fiber grating filter as a gain equalizer. See, for example, N. S. Bergano et al., xe2x80x9c100/Gb/s WDM Transmission of Twenty 5 Gb/s NRZ Data Channels Over Transoceanic Distances Using a Gain Flattened Amplifier Chainxe2x80x9d, Th. A. 3. 1, ECOC""95.
A third proposed method is to use a Mach-Zehnder-type gain equalizer with an Al-low-density optical amplifier. See Kazuhiro Oda et al., xe2x80x9c16-Channelxc3x9710-Gbit/s Optical FDM Transmission Over a 1000 km Conventional Single-Mode Fiber Employing Dispersion-Compensating Fiber and Gain Equalizationxe2x80x9d, OFC""95, PD22-1 to PD-22-5.
In each of the above-described proposed methods, generally, a gain equalizer has reverse transmission characteristics in relation to the amplifier characteristics of an EDFA. Thus, the gain equalizer compensates for the amplifier characteristics to produce a flatter amplifier gain in a narrow band, thereby obtaining a flat transmission band and reducing dispersion of optical SNR. The WDM signal is transmitted in the narrow band.
Generally, the narrow band is set as a 10 nm band of 1550 to 1560 nm. The narrow band is limited in size since, generally, the above-described proposed methods can only compensate for the gain of an EDFA around a single gain peak in the gain characteristics of the EDFA, or along a small portion of positive or negative gain slopes of the gain characteristics of the EDFA. Therefore, the narrow band for transmission allowed by the above-described proposed methods is too narrow to transmit a WDM signal which includes a relatively large number of individual signal lights.
Accordingly, it is an object of the present invention to provide a gain equalizer which will sufficiently flatten gain versus wavelength characteristics of an EDFA over a relatively large band when tens of repeaters are connected in series. Preferably, such a relatively large band can include, for example, several 10-nm bands of 1.53-xcexcm band to 1.56-xcexcm band.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The foregoing objects of the present invention are achieved by providing an apparatus, such as a gain equalizer, for equalizing gain versus wavelength characteristics of an optical amplifier. The gain versus wavelength characteristics have first and second gain peaks in a wavelength band with a wavelength difference between the first and second gain peaks. The apparatus includes first and second optical filters connected to the optical amplifier and having first and second transparency characteristics, respectively. The first and second transparency characteristics are periodic waveforms having periods related to the wavelength difference between the first and second gain peaks. The period of the waveform of the first transparency characteristic is different from the period of the waveform of the second transparency characteristic.
Objects of the present invention are also achieved by providing an optical communication system which includes an optical amplifier and first and second optical filters. The optical amplifier has gain versus wavelength characteristics with first and second gain peaks in a wavelength band and a wavelength difference between the first and second gain peaks. The first and second optical filters have first and second transparency characteristics, respectively. The first and second transparency characteristics are periodic waveforms having periods related to the wavelength difference between the first and second gain peaks. The period of the waveform of the first transparency characteristic is different from the period of the waveform of the second transparency characteristic. The first and second optical filters each filter either an input signal to the optical amplifier or an output signal of the optical amplifier.
Objects of the present invention are also achieved by providing a method for equalizing gain versus wavelength characteristics of an optical amplifier which amplifies an input signal in accordance with the gain versus wavelength characteristics, to produce an output signal. The gain versus wavelength characteristics have first and second gain peaks in a wavelength band with a wavelength difference between the first and second gain peaks. The method includes the steps of (a) filtering either the input signal or the output signal with a first transparency characteristic, and (b) filtering either the input signal or the output signal with a second transparency characteristic. The first and second transparency characteristics are periodic waveforms having periods related to the wavelength difference between the first and second gain peaks. The period of the waveform of the first transparency characteristic is different from the period of the waveform of the second transparency characteristic.
Objects of the present invention are further achieved by providing an additional method for equalizing gain versus wavelength characteristics of an optical amplifier. The gain versus wavelength characteristics have first and second gain peaks in a wavelength band with a wavelength difference between the first and second gain peaks. The method includes the steps of (a) branching a light signal into first and second signals, (b) filtering the first signal with a first transparency characteristic, (c) filtering the second signal with a second transparency characteristic, and (d) combining the filtered first and second signals into a combined signal which is amplified by the optical amplifier. The first and second transparency characteristics are periodic waveforms having periods related to the wavelength difference between the first and second gain peaks. Moreover, the period of the waveform of the first transparency characteristic is different from the period of the waveform of the second transparency characteristic.
In addition, objects of the present invention are achieved by providing a method which includes the steps of (a) branching an output signal of an optical amplifier into first and second signals, (b) filtering the first signal with a first transparency characteristic, (c) filtering the second signal with a second transparency characteristic, and (d) combining the filtered first and second signals into a combined signal. As described above, the first and second transparency characteristics are periodic waveforms having periods related to the wavelength difference between first and second gain peaks of the gain versus wavelength characteristics of the optical amplifier. Moreover, the period of the waveform of the first transparency characteristic is different from the period of the waveform of the second transparency characteristic.